SSL/TLS Strong Encryption: How-To

The solution to this problem is trivial
and is left as an exercise for the reader.

-- Standard textbook cookie

How to solve particular security problems for an SSL-aware
webserver is not always obvious because of the interactions between SSL,
HTTP and Apache's way of processing requests. This chapter gives
instructions on how to solve some typical situations. Treat it as a first
step to find out the final solution, but always try to understand the
stuff before you use it. Nothing is worse than using a security solution
without knowing its restrictions and how it interacts with other systems.

httpd.conf

This facility is called Server Gated Cryptography (SGC) and requires
a Global ID server certificate, signed by a special CA certificate
from Verisign. This enables strong encryption in 'export' versions of
browsers, which traditionally could not support it (because of US export
restrictions).

When a browser connects with an export cipher, the server sends its Global
ID certificate. The browser verifies this, and can then upgrade its
cipher suite before any HTTP communication takes place. The problem
lies in allowing browsers to upgrade in this fashion, but still requiring
strong encryption. In other words, we want browsers to either start a
connection with strong encryption, or to start with export ciphers but
upgrade to strong encryption before beginning HTTP communication.

Obviously, a server-wide SSLCipherSuite which restricts
ciphers to the strong variants, isn't the answer here. However,
mod_ssl can be reconfigured within Location
blocks, to give a per-directory solution, and can automatically force
a renegotiation of the SSL parameters to meet the new configuration.
This can be done as follows:

# be liberal in general
SSLCipherSuite ALL:!ADH:RC4+RSA:+HIGH:+MEDIUM:+LOW:+SSLv2:+EXP:+eNULL

When you know all of your users (eg, as is often the case on a corporate
Intranet), you can require plain certificate authentication. All you
need to do is to create client certificates signed by your own CA
certificate (ca.crt) and then verify the clients against this
certificate.

httpd.conf

# require a client certificate which has to be directly
# signed by our CA certificate in ca.crt
SSLVerifyClient require
SSLVerifyDepth 1
SSLCACertificateFile conf/ssl.crt/ca.crt

The key to doing this is checking that part of the client certificate
matches what you expect. Usually this means checking all or part of the
Distinguished Name (DN), to see if it contains some known string.
There are two ways to do this, using either mod_auth_basic or
SSLRequire.

The mod_auth_basic method is generally required when
the certificates are completely arbitrary, or when their DNs have
no common fields (usually the organisation, etc.). In this case,
you should establish a password database containing all
clients allowed, as follows:

These examples presume that clients on the Intranet have IPs in the range
192.168.1.0/24, and that the part of the Intranet website you want to allow
internet access to is /usr/local/apache2/htdocs/subarea.
This configuration should remain outside of your HTTPS virtual host, so
that it applies to both HTTPS and HTTP.

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